X0609 4 2026-10-05 0 0002070849 BillionToOne, Inc. BLLN 0002087127 Tsao David false C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK CA 94025 1 1 0 0 Chief Technology Officer 1 Class A Common Stock 2026-10-05 4 M 0 10000 2.80 A 11000 D Class A Common Stock 2026-10-05 4 M 0 10000 2.80 A 21000 D Class A Common Stock 2026-10-05 4 S 0 866 105.9004 D 20134 D Class A Common Stock 2026-10-05 4 S 0 1909 107.195 D 18225 D Class A Common Stock 2026-10-05 4 S 0 4284 108.393 D 13941 D Class A Common Stock 2026-10-05 4 S 0 2921 109.0031 D 11020 D Class A Common Stock 2026-10-05 4 S 0 20 110.08 D 11000 D Class A Common Stock 2026-10-05 4 S 0 555 105.5587 D 10445 D Class A Common Stock 2026-10-05 4 S 0 400 106.1038 D 10045 D Class A Common Stock 2026-10-05 4 S 0 2200 107.4705 D 7845 D Class A Common Stock 2026-10-05 4 S 0 4916 108.5342 D 2929 D Class A Common Stock 2026-10-05 4 S 0 1829 109.1445 D 1100 D Class A Common Stock 2026-10-05 4 S 0 100 110.09 D 1000 D Stock Option (right to buy) 2.80 2026-10-05 4 M 0 10000 2.80 D 2031-06-07 Class A Common Stock 10000 546667 D Stock Option (right to buy) 2.80 2026-10-05 4 M 0 10000 2.80 D 2031-06-07 Class A Common Stock 10000 536667 D The transactions reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on June 5, 2026. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $105.505 to $106.350 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $106.730 to $107.700 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $107.7354 to $108.710 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $108.740 to $109.480 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $104.880 to $105.720 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $105.890 to $106.180 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $107.010 to $107.970 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $108.040 to $109.030 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $109.040 to $109.500 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The options are fully vested and exercisable. /s/ Thomas P. Lynch, Attorney-in-Fact 2026-10-07